Just like libmutter-clutter, and libmutter, mark exported symbols with
an COGL_EXPORT macro. This removes the .map and .map.in files previously
used, containing a list of semi private symbols. This symbol was out of
date, i.e. pointed to non-existing symbols, and was also replaced with
COGL_EXPORT macros.
unit_test_* symbols are exported by the help of the unit test defining
macro. test_* symbols are no longer supported as it proved unnecessary.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1059
